Various sensors have been used to detect the presence of compounds in a gas sample, e.g., U.S. Patent Publication Nos. 2009/0054799 and 2013/0115706, U.S. Pat. No. 5,787,885, Figaro TGS 822 Sensor (http://www.figarosensor.com/products/822pdf.pdf), Breath acetone monitoring by portable Si:WO3 gas sensors” (Analytica Chimica Acta 738 (2012) 69-75). However, some acetone sensing devices require higher than ambient operating temperatures. In some cases, the sensors operate at very high temperatures, between 100° C. and 500° C., see Wang, et al., Chem. Mater., 20:4794-4796 (2008); Wang et al., Sensor Letters, 8:1-4 (2010); Righettoni et al., Anal. Chem., 82:3581-3587 (2010). For environmental gas sensors, a heated sensor chip is placed in a mesh enclosed chamber within the device. In order to protect the temperature field around the heated sensor chip and to maintain the temperature of the sensor, the analyte is only allowed to reach the sensor by diffusion. However, when analyzing compounds from breath, the analyte comes in contact with the sensor by a flow mechanism (i.e., an exhaled breath or mechanical discharge), which alters the pre-established equilibrium around the heated sensor. As a result of their small size and the temperature difference between the sensor and exhaled breath, flow over the sensor causes heat loss by convection. Exhaled breath also has high moisture content, which can cool down the sensor.
When a gas sample at body temperature directly contacts the sensor surface, the sensor surface cools down quickly by convection which can result in inaccurate measurements. Moreover, the flow rate of human-exhaled breath varies from person to person so that the convection heat loss cannot be easily predicted. The heat loss can cause the temperature of the sensor to drop below the sensor's operating temperature, requiring the sensor to heat up again to operate. These temperature fluctuations can result in inconsistent and/or inaccurate results.
Therefore, there remains a need in the art for a breath sensor apparatus that reduces temperature fluctuation caused by flow rates in order to obtain more consistent and accurate results. The disclosed apparatus provides more consistent and accurate results for detecting volatile organic compounds from exhaled breath by reducing the flow rate variation through flow redistribution using a fixed element.